The field of the present invention is timing mechanisms for the exhaust ports of two-cycle engines.
Two-cycle engines typically employ the upper edge of a piston as the means for timing the opening and closing of the exhaust ports. In such engines, the exhaust passage may be tuned such that a reflected wave of pressure initiated by the opening of the exhaust ports can force unburned air fuel mixture, trailing the exhaust gases through the exhaust passage, back into the cylinder just prior to closure of the exhaust porting. This tuning of the exhaust is specifically effective at a limited range of engines speed. Often power can drop off remarkably when the engine speed does not fall within the tuned range.
To broaden the effective power range for two-cycle engines, exhaust timing control devices have been employed which provide an apparent upper timing edge to the exhaust porting that may be moved upwardly or downwardly depending on engine speed. In this way, power can be realized across a broader range of engine speeds. As the timing for both the opening and closing of the exhaust porting is changed through movement of a valve mechanism, the timing can better employ a pressure wave in the exhaust passage.
The speed at which two-cycle engines operate and the harsh environment of the exhaust of such engines have resulted in substantial design problems in devising such exhaust timing control devices. The devices must respond quickly to changes in engines speed, they must be capable of withstanding a wide temperature range from cold engine starts to continuous high power operating conditions and they must continue to operate in an environment having a tendency to accumulate deposits of carbon in areas not subjected to direct high temperature exhaust flow.
The foregoing environment requires mechanisms which will not quickly erode under the impingement of high temperature exhaust flow and yet be sufficiently light to respond quickly to changing engine speeds. The harsh environment also requires high heat transfer at thin walled sections, avoidance of direct impingement on such sections and the avoidance where possible of thin walled sections themselves. In addition, the harsh environment encountered by such timing control devices is enhanced by sealing mechanisms which prevent exhaust flow from depositing carbon in areas where the relative movement of parts would be inhibited by carbon build-up.
One prior device employed for exhaust timing control is disclosed in Japanese Utility Model Publication No. 36047/1972. In this device, the thin components exposed to high temperature exhaust gas can be adversely affected. A problem with increasing the size, and correspondingly the weight, of such components to overcome the thermal loads is that the devices become too heavy to respond properly to rapid changes in engine speed without correspondingly larger components employed for driving the valve. This can in turn reduce the effectiveness of the exhaust passage as well.
In systems where more substantial components may be arranged to avoid such problems with heat, by being located in recesses of the exhaust passage for example, problems can then develop in the creation of thin wall sections in the cylinder, principally around the exhaust port where such recesses approach the cylinder wall. Where thin walled areas exist, heat is less able to be conducted away to cooling devices on the engine. Such systems employing recesses and the like also can have areas where carbon build-up is detrimental to the operation of the system. However, sealing of such areas is difficult because of the high temperature environment and because of the need for clearances to accommodate thermal expansion and contraction of the components. Thus, substantial design considerations are required in the construction of such exhaust timing control devices.